# std::execution::seq, std::execution::par, std::execution::par_unseq, std::execution::unseq

From cppreference.com
< cpp‎ | algorithm

Defined in header `<execution>` |  |
inline constexpr std::execution::sequenced_policy seq { /* unspecified */ }; |  | (since C++17)
inline constexpr std::execution::parallel_policy par { /* unspecified */ }; |  | (since C++17)
inline constexpr std::execution::parallel_unsequenced_policy par_unseq { /* unspecified */ }; |  | (since C++17)
inline constexpr std::execution::unsequenced_policy unseq { /* unspecified */ }; |  | (since C++20)

The execution policy types

- std::execution::sequenced_policy,
- std::execution::parallel_policy,
- std::execution::parallel_unsequenced_policy, and
- std::execution::unsequenced_policy

have the following respective instances:

- `std::execution::seq`,
- `std::execution::par`,
- `std::execution::par_unseq`, and
- `std::execution::unseq`.

These instances are used to specify the execution policy of parallel algorithms, i.e., the kinds of parallelism allowed.

Additional execution policies may be provided by a standard library implementation (possible future additions may include `std::parallel::cuda` and `std::parallel::opencl`).

### Example

Run this code

    #include <algorithm>
    #include <chrono>
    #include <cstdint>
    #include <iostream>
    #include <random>
    #include <vector>

    #ifdef PARALLEL
    #include <execution>
        namespace execution = std::execution;
    #else
        enum class execution { seq, unseq, par_unseq, par };
    #endif

    void measure([[maybe_unused]] auto policy, std::vector<std::uint64_t> v)
    {
        const auto start = std::chrono::steady_clock::now();
    #ifdef PARALLEL
        std::sort(policy, v.begin(), v.end());
    #else
        std::sort(v.begin(), v.end());
    #endif
        const auto finish = std::chrono::steady_clock::now();
        std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(finish - start)
                  << '\n';
    };

    int main()
    {
        std::vector<std::uint64_t> v(1'000'000);
        std::mt19937 gen {std::random_device{}()};
        std::ranges::generate(v, gen);

        measure(execution::seq, v);
        measure(execution::unseq, v);
        measure(execution::par_unseq, v);
        measure(execution::par, v);
    }

Possible output:

    // online GNU/gcc compiler (PARALLEL macro is not defined)
    81ms
    80ms
    79ms
    78ms

    // with g++ -std=c++23 -O3 ./test.cpp -ltbb -DPARALLEL
    165ms
    163ms
    30ms
    27ms

### See also

sequenced_policyparallel_policyparallel_unsequenced_policyunsequenced_policy (C++17)(C++17)(C++17)(C++20) | execution policy types (class)

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